The present invention relates to a compliant end effector for an industrial robot, and more particularly to an end effector that compliantly holds an implement with a tool and allows the implement to pivot in any direction relative to a given reference line when the tool engages a workpiece in a robotic manufacturing application.
Many industries use robotics to speed up manufacturing, improve product quality, reduce costs and provide a safer working environment for employees. Parts are robotically worked by securing them to a work station so that the part is located at specific coordinates. The robot is then programmed to move along predetermined paths of travel, and to rotate, twist and turn at prescribed points along these paths. For each path of travel, the robot is programmed to move between specific points. The path may be linear or have a defined arcuate shape, such as a circle, ellipse, parabola, or portion thereof. A tool secured to the robotic arm performs desired tasks on the part as the robot moves along its path of travel. The robot will follow the programmed path of travel without deviation. The robot performs the same tasks at the same places on each part passing through the work station. Such robotic work stations process parts quickly and manufacture them to relatively high degrees of tolerances.
A problem with robotic manufacturing is that the robots must be programmed to take into account every surface of the workpiece being machined. While the programming process can be fairly easy for parts with relatively simple shapes, programming becomes arduous for more complicated part shapes. Conventional robots and their end effectors do not compensate for small, intended deviations in an otherwise uniform surface of the workpiece. For example, a cast or molded part may have several flat surfaces with small abutments, recesses or screw holes for aligning or joining that part to another part. These abutments and the walls of the screw holes project outwardly from the otherwise uniform, linear or arcuate surface of the workpiece. The robot must be programmed to adjust for each of these deviations or the projection can be ground or cut out of the workpiece. The programmer must account for every intended deviation in the otherwise uniform surfaces of the workpiece, or risk producing a potentially defective product. Writing programs that take into account every intended deviation in a complex part shape is tedious, time consuming, expensive and prone to mistake. Several test runs may have to be performed before the program is ready for production.
Another problem with conventional robotic manufacturing is that the robot will not compensate for any misalignment of the workpiece at the work station. This is particularly problematic when manufacturing large, heavy workpieces, such as a vehicle transmission housing. Such workpieces are difficult to move into a precise orientation and coordinates. The robot will also not compensate for small imperfections in the workpiece, such as any warpage in its surface. The robot will gouge, undercut or miss the workpiece due to any such misalignment or imperfection.
A further problem with robotic manufacturing is that each change in path of the robotic arm can cause imprecision in the finished part due to the tolerances associated with the movements of the robotic arm. Complex parts with a variety of surfaces can be problematic because the robotic arm must travel along many different paths of travel. Small amounts of tolerance can accumulate to produce a significant imprecision in the workpiece.
A still further problem with robotics is that the robotic arms are designed to meet specific industry needs, and thus each arm has specific weight and torque capacities. Should the end effector and implement exceed these limits during use, the robotic arm can malfunction, operate inappropriately, wear out more quickly, or break down. The lighter the end effector and the more balanced it is when holding an implement, the heavier and more powerful the implement that can be held by the robotic arm. Accordingly, end effectors should have a compact, lightweight and balanced design.
A still further problem with robotics is designing an end effector suitable for compliantly matching a variety of differently shaped parts. The end effector must be able to compensate for deviation and misalignments that may arise along any horizontal, vertical, angled, or arcuate path of travel. Conventional end effectors may allow the tool to compliantly engage the workpiece when the end effector and tool are upright, but not when they are turned sideways or upside down. The end effector may become jammed or become to stiff or too lose when turned sideways or upside down. The amount of compliance may also deviate depending on the orientation of the end effector and tool. The workpiece will need to be repositioned one or more times to accommodate the limitation of the end effector. Such an end effector has a greatly reduced value in a commercial manufacturing operation.
A still further problem with conventional compliant end effectors is that it is difficult to easily and securely attach an implement to the end effector. Implements are frequently heavy, bulky and awkward to handle, which can result in misalignments. Workers can also bump or drop the implement, causing damage to the implement or injuring themselves or a coworker.
The present invention is intended to solve these and other problems.
The present invention relates to a compliant end effector for securing an implement or tool such as a spindle to an arm of an industrial robot. The end effector has an internal passageway that extends completely through the end effector for receiving the spindle. The end effector includes a mounting assembly, a gripping assembly, a compliant assembly and a biasing assembly. The mounting assembly has a mounting bracket that rigidly secures the end effector to the robotic arm. The gripping assembly has a support bracket that rigidly secures the spindle to the end effector. The compliant assembly joins the mounting assembly to the gripping assembly, and includes an internal collar with two sets of opposed pivot pins that form first and second pivot axes. The biasing assembly includes a sponge rubber biasing ring with a number of uniformly spaced springs that combine to bias the spindle into a normal biased position. The spindle has a reference axis that forms a reference line for the end effector when in this biased position. The collar and biasing ring also have openings that form a part of the passageway. The two pivot axes allow the spindle to pivot in any direction relative to the reference line through 360xc2x0 around the reference line. The biasing assembly includes a stiffness adjustment assembly that produces a pre-load condition to adjust the amount of force needed to pivot the spindle out of its normal biased position. As the robotic arm moves the implement along a uniform path of travel, the tool engages a workpiece with a substantially uniform surface or edge and cuts away unwanted burs or other unintended discontinuities from that surface or edge. However, the tool compliantly pivots relative to the reference line when the tool encounters a desired projection in the workpiece so that the tool does not gouge or cut away that desired projection.
One advantage of the present compliant end effector invention is its simplicity of use. The robotic arm does not need to be programmed to take into account every intended deviation in the surface of the workpiece. The end effector securely holds the spindle so that the tool will compliantly engage the workpiece as the spindle moves along a uniform path of travel. This dramatically reduces the amount of programming necessary to machine more complicated parts. Casted and molded parts with relatively uniform surfaces with a number of abutments, recesses and screw holes can be easily processed using a robotic arm with the present compliant end effector. By setting the end effector to a desired amount of stiffness, the end effector will engage and ride over the projections formed by these abutments and screw holes. The robot need only be programmed to move in uniform paths corresponding to each of the relatively uniform surfaces of the workpiece. Each intended deviation need not be individually programmed. Accordingly, the time and cost to program of the robotic arm is greatly reduced, particularly for more complex workpieces.
Another advantage of the present compliant end effector invention is its adaptability. The end effector allows the robot to compensate for the slight misalignment of the workpiece at the work station. This is particularly useful when manufacturing large, heavy workpieces, such as a vehicle transmission housing. The compliant end effector can compensate for slight misalignment of the workpiece from its desired orientation. Similarly, the end effector can compensate for small imperfections in the workpiece, such as any warpage in the surface of the workpiece. The robot will not gouge, undercut or miss the workpiece due to such misalignment of imperfection.
A further advantage of the present compliant end effector invention is its precision. Fewer changes in the path of the robotic arm are required for most workpieces. This helps reduce the imprecision in the finished part due to the tolerances associated with the robotic arm. Complex parts with several surfaces can be more easily programed because the robotic arm travels along fewer paths to machine the generally uniform surfaces of the workpiece. A more precise part is produced because fewer movements of the robotic arm are required to complete a job.
A still further advantage of the present compliant end effector invention is its speed. The end effector improves the speed of a robot and reduces the time needed to perform a manufacturing operation. Robots take time to calculate desired paths of travel based on program points entered by the robot programmer. Robots using the compliant end effector need fewer program points to guide the end effector along its intended paths of travel during a machining operation. The reduction in program points speeds up the robots ability to calculate the desired paths of travel, and thus enables the robot to perform a manufacturing operation more quickly. This increased speed is multiplied by every workpiece manufactured.
A still further advantage of the present compliant end effector invention is its stability. The biasing assembly includes a biasing ring that combines with a number of springs to produce a stable biasing mechanism. The biasing ring produces about half the biasing load of the biasing assembly, and the springs 211 produce the remainder of the biasing load. The biasing ring also serves as a damper. Should the spindle or tool experience any chatter or other vibrations during operation. The biasing ring dampens these vibrations so that they do not multiply together and damage the tool or workpiece.
A still further advantage of the present end effector invention is its versatility. The end effector is relatively light in weight so that it can be used with a variety of implements on a variety of robots without exceeding their weight and torque capacities of the robotic arms. The end effector is designed to hold an implement in a compact and balanced manner. The implement passes through the center of the end effector so that a large portion of the weight of the implement extends from the front of the end effector and a large portion of the weight of the tool extends from the rear of the end effector. For example, end effector is capable of holding an industrial spindle having a length of about two feet so that the center of gravity of the end effector and implement is about six inches from the distal end of a robotic arm and substantially in line with the centerline of the robotic arm. The compact nature of the end effector provides the additional benefit of reducing its weight. This compact, light weight and balanced design allows the end effector to be used with a wide range of robotic arms, as well as with heavier and more powerful implements without exceeding the capacity of the robotic arm.
A still further advantage of the present end effector invention is its consistency. The end effector provides a consistent amount of compliance to the tool independent of its orientation. The end effector can be upright, sideways or inverted without significantly changing the amount of compliance provided to the tool. A consistent amount of compliance is provided when the robotic arm moves along different horizontal, vertical, angled, or arcuate paths of travel. Thus, this versatile end effector can be used in a robotic manufacturing process for a wide variety of differently shaped parts. The robotic arm is free to move about the part without repositioning the part each time a different side is machined.
A still further advantage of the present end effector invention is the ease with which an implement can be installed. The implement is simply inserted into the passageway of the end effector so that the rear housing of the implement abuts a gripping collar of the gripping assembly. Once inserted, the worker can let go of the implement as it is now supported by the end effector. Four clamping bolts are then tightened to form a compression fit between the gripping collar and the implement to rigidly secures the implement to the end effector. Accordingly, even heavy, bulky and awkward to handle implements can be secured to the end effector with relative ease.